u/FinishingDutch Sorry, I never saw this earlier.

Maybe I shouldn't say 'trick' as in something that you can apply everywhere, it was particular to the type of frames I got. These, with a spring-loaded clamping action around the outline and a thin kind of flexy matte "glass" plastic layer: https://www.amazon.com/gp/product/B07D9QJZKD/

I was able to mount the frame on the wall first. Then with the side edge channels and top channel open, get one edge of the poster cleanly into the bottom edge, behind the film, and clamp it down. (The film is otherwise sagging out against my face basically, like at a 30deg angle to the wall, so its not 'crushing' the still rolled tube of poster.

Then slowly run my fingers up the edge, unrolling and using my chest and whatever I could to try and mush the part of the matte film over the unrolled portion of the poster back, until i got to the top and was able to clamp the top edge and hold it tight, then get the sides down.

Not gonna lie, more than once it all slipped out of my hands on the way 'up' unrolling, or the bottom clamped edge popped out and window-shaded up...

Not much of a trick in retrospect.